Hey, during preseason I have been running smite thresh support as I find it is very strong and I list the reasons why in my pro's and con's :) This will be a straight forward guide, I will try to go in-depth into some thresh specific stuff later but this is focused on the reasons why smite support will/can work.

The pro's and cons of smite thresh:

Pro's:
A Summoner Spell is up every sixty seconds
You get a nice slow
You get a bit of regular true damage
You can smite minions in lane for surprise Q's on various Supports.
You can help secure monsters
You can collect mini-buffs for yourself(Such as the free red trinket from smiting a raptor, the free roaming ward from smiting wolf*you do not need to kill them only smite them*)
You can leash your jungler extremely well and secure objectives easily including the 5 dragons which pretty much win you the game.
Jungle item enchant makes you quite tanky and gives you free tenacity when coupled with an elixer of iron will give you 51% tenacity! All cc is halved!
Can smite minions with relic shield from a distance and it proc's!

Con's:
You miss out on exhaust or ignite
You may be called a troll etc until you start winning
If you lose you could get reported
It can be a blow to your gold income depending on the way you build.

I am actually not going to go over the items, I feel like it's straight forward my reasoning for taking the items I take can be justified once you try it out, you get alot of health, free red trinket or roaming wards, great leash with jungler, secure objectives easily, can smite minions to proc your relic shield from a distance(especially late game am I right!?) Slows, speed up for your team that you miss out from by not going talisman, tenacity ~

Now onto thresh mechanics.

When playing as Thresh you can click to face and move one way and then hook another direction simply but hovering your mouse over there, it will catch people off guard especially in high elo, it is called the "toyz mechanic" feel free to youtube it.

When playing against people with gap closing abilities you can USUALLY interupt them, for example:

You are facing a leona and she goes to E onto you or your adc, you quickly flay backward and she is stopped and it goes on cooldown

Example 2:

You are facing a tristana who jumps away, she goes into the air and you flay and she comes crashing back down with it on full CD.

When playing as thresh and your jungler is coming in for a gank, DO NOT STAND THERE AND WAIT FOR HIM TO GO IN. You take charge, hook in, lantern, flay, smite them etc YOU take charge. You have all the cc and mobility and you are able to allow your jungler to come to you in a matter of seconds, use this!!!! So what if they get away because you used all your cc at the start, you'll blow summoners and you could even get kills. You create pressure! That flash you just blew? Type it in chat "Trist flash *5 minutes after the time she blew it*" So if it's 2:30 when your jungler ganks and she flashes at 2:33 then you go "Trist flash 7:33" your jungler now can come back at 6:33 and be like "ok 1 min till flash let's kill her" or come back at 7:33 to blow it again.

All in all I really like smite support, especially on thresh and blitz. I listed all the reasons why and I really hope you try it out and enjoy it, if you liked anything please give it a rating and comment why, if you have any sort of feedback at all even negative please comment, I'll take it all :) I am a newb to guides so I am sorry if it does not look fancy, my intention here isn't to make the best looking guide but to make an informative one that helps players out and shifts the meta to smite support! Because I seriously think it's better than exhaust.